Currently, the width of transistors is getting continuously narrower due to ongoing transistor miniaturization aimed at achieving ever larger scale integration of semiconductor devices.
Hot carrier effects inside the transistor worsen as transistors shrink. When the length of a channel becomes much shorter while the rated operating voltage decreases little, the horizontal electric field becomes very high near the drain region. This can degrade electrical characteristics of the drain region through the generation of hole and electron pairs. The hot carrier effect produces holes which move toward the substrate.
The electrons move toward the gate, and may become trapped below the gate oxide film or a spacer. These trapped electrons have a cumulative effect on the threshold voltage of the transistor.
The hot carrier effect becomes more severe as higher electric fields are applied to the channel regions in the semiconductor substrate. The electric field gets higher because the power supply or rated operating voltage is relatively constant but the channel length is getting much shorter. The shorter the channel between source and drain, the worse the hot carrier effects.
To overcome the hot carrier effect, transistors may use an LDD (Lightly Doped Drain) structure. The structure has a graded junction where ion implantation density in the source and drain region is low around edges of the gate electrode and high around center of the gate electrode, thereby reducing and abrupt change in the voltage gradient, and reducing the electric field.
However, a transistor with an LDD structure suffers a short channel effect since the channel length is getting continuously shorter due to larger scale integration of semiconductor devices. Dopants in the LDD region diffuse into the channel and may create a high electric field in the channel edge near the drain, which causes the hot carrier effect that degrades performance of the transistor.
Moreover, during the operation of the transistor, impurities in source and drain regions diffuse laterally. The transistor may become more susceptible to a punch-through effect. It is difficult to formulate a process of ion implantation for preventing the punch-through effect. Also, it is difficult to control the threshold voltage when the channel length and the ion density control are not accurately controlled.